optima compact unable to reset entry time?

i have a optima compact fitted at home. i had to turn off the electric to carry out other work when i switched the power back on the exit time is ok 30 seconds but the entry time is a problem when the property is entered the alarm goes off straight away;; i know the engineers code and have the instructions but no matter how i try i cannot sort it (someone said it could be the battery ? ) help!!!!